Digital subscriber line (DSL) technology allows for high-bandwidth networking connections to be made over ordinary copper telephone lines. Traditional phone service typically relies on unshielded twisted pair (UTP) copper lines to connect homes and small businesses to the communications network operated by the telephone company (TELCO). Every one of these networks includes a central offices (CO) that services a defined region, with each CO responsible for connecting and routing calls directed to sites that reside both internal to and external of the network.
Branching out from the central office are numerous remote terminals (RT) located throughout the region being served by the CO, with each RT providing the phone service for the subscribers located within a specific area or neighborhood. One of the primary components that make up a remote terminal (RT) is a pair gain system, also known as a derived carrier system, or digital loop carrier system. In simplest terms, the pair gain system provides the TELCO with the capability to carry multiple services over a lesser number of lines, for example, five conversations over one telephone line. The pair gain system also is responsible for generating the dial tone signal one hears when they first pick up a telephone handset, indicating that an active connection is present.
FIG. 1 depicts a typical telephone connection between a pair gain system 1 of a remote terminal (RT) (not shown) and a subscriber 3. As indicated in FIG. 1, the connection between the pair gain system 1 and the subscriber 3 is not accomplished directly, but instead in two legs. The first connection 1a exists between the pair gain system 1 and a cross-connect block 2, while the second connection 2a is made between the cross-connect block 2 and the subscriber 3. As indicated by its name, the purpose of the cross-connect block 2 is to allow easy matching and connecting of two or more connections to one another, thereby facilitating the addition or removal of phone services to or from the subscriber. In most applications, the number of subscriber connections at the cross-connect block is greater than the number of pair gain system connections; typically the ratio of subscriber connections to pair gain connections is 2:1.
To further illustrate the use of the cross-connect block 2, consider the following example involving a typical modern day residence. New homes are often pre-wired to handle multiple telephone lines, i.e. six lines, to allow for future expansion. In this situation, there would be six connections running between the subscriber's house 3 and the cross-connect box 2. However, if the subscriber only has two active phone lines, then only two connections representing the active circuits would run from the pair gain system 1 to the cross-connect box 2. Later, if the subscriber wishes to add a new telephone line for a fax machine, a technician would have to run a new connection between the pair gain system 1 and the cross-connect block 2, matching the new connection at the cross-connect block 2 to the appropriate connection already present that runs to subscriber's house 3.
Note that each of connections 1a and 2a, along with the connections discussed below, although drawn as single lines in the figures, actually represent a cable pair, such as, for example, typical unshielded twisted pair (UTP) copper lines. For the remainder of the application, the terms “connection”, “cable pair”, and “line” should be considered interchangeable.
As DSL technology is relatively new compared to typical telephony communication involving analog signals, many of the remote terminals (RT) that are part of a telephone company's (TELCOs) network were not designed to allow easy incorporation of newer technology such as DSL. Accordingly, the TELCOs have had to develop ways to effectively provide DSL service to their subscribers utilizing the existing equipment on the network.
FIG. 2 depicts a typical approach to incorporating DSL service with a remote terminal (RT). The dial tone signal generated by the pair gain system 1 is directed to a splitter 5 via connection 1a. The splitter 5 also receives a connection 4a from a DSL system 4. The DSL system 4 includes the equipment necessary for processing and directing the data signals back and forth between the subscriber 3 and a digital subscriber line access multiplexer (DSLAM) (not shown). The DSLAM, which is operated by a service provider, takes all of the subscribers' DSL connections and aggregates them onto a single, high-density connection to the Internet. For the current illustrative example involving the integration of DSL at an RT, the DSL system may be physically mounted inside the cabinet housing the RT, or placed in its own cabinet mounted onto or next to the RT depending on factors such as size limitations and ease of access.
The role of the splitter 5 is to combine the lower frequency signal from the pair gain system 1 with the higher frequency DSL data signal in such a way that they do not interfere with one another. Similarly, the splitter 5 must also be capable of separating the signal sent by the subscriber 3 back into its two constituent components and then direct them back to the appropriate system. In FIG. 2, the splitter 5 is depicted as an independent component separate from the DSL system 4. Alternatively, the splitter 5 may be incorporated into the DSL system 4.
The combined signal produced by the splitter 5 is delivered to the cross-connect block 2 over connection 5a, where it is then directed to the subscriber 3 over connection 2a. Subscribers 3 can then access the higher frequency DSL signal by means of a DSL modem connected between their computing device and the telephone line(s) running throughout their residence. At the same time, standard telephones continue to have access to the lower frequency analog signals also routed over the line(s).